In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of oxides of nitrogen (NOx) and carbon monoxide (CO). Lower combustion emissions can often be attributed to a more efficient combustion process, with specific regard to fuel injector location and mixing effectiveness.
Early combustion systems utilized diffusion type nozzles, where fuel is mixed with air external to the fuel nozzle by diffusion, proximate the flame zone. Diffusion type nozzles have been known to produce high emissions due to the fact that the fuel and air burn stoichiometrically at high temperature to maintain adequate combustor stability and low combustion dynamics.
An enhancement in combustion technology is the utilization of premixing, such that the fuel and air mix prior to combustion to form a homogeneous mixture that burns at a lower temperature than a diffusion type flame and produces lower NOx emissions. Premixing fuel and air together before combustion allows for the fuel and air to form a more homogeneous mixture, which will burn more completely, resulting in lower emissions. However, in this configuration the fuel is injected in relatively the same plane of the combustor, and prevents any possibility of improvement through altering the mixing length.
Premixing can occur either internal to the fuel nozzle or external thereto, as long as it is upstream of the combustion zone. An example of a premixing combustor 100 of the prior art is shown in FIG. 1. The combustor 100 is a type of reverse flow premixing combustor utilizing a pilot nozzle 102, a radial inflow mixer 104, and a plurality of main stage mixers 106 and 108. The pilot portion of the combustor 100 is separated from the main stage combustion area by a center divider portion 110. The center divider portion 110 separates the fuel injected by the pilot nozzle 102 from the fuel injected by the main stage mixers 106 and 108. While the combustor 100 of the prior art has improved emissions levels and ability to operate at reduced load settings, analysis and testing has demonstrated the onset of thermo acoustic dynamics due to symmetries generated in the burner as a result of the burner geometry, such as the center divider portion.
As one skilled in the art understands, mechanisms that cause thermo-acoustic instabilities are coherent structures generated by the burner. One type of combustor known to exhibit such instabilities is a combustor having a cylindrical shape. What is needed is a system that can provide flame stability and low emissions benefits at a part load condition while also reducing thermo-acoustic instabilities generated by coherent flame structures.